The emphatic victory ensured Stefanos Tsitsipas a top-two place in the group and left Alexander Zverev, world number one Rafael Nadal and Daniil Medvedev scrapping to join him in Saturday's semi-finals.
Debutant Stefanos Tsitsipas crushed defending champion Alexander Zverev 6-3, 6-2 on Wednesday to book his place in the last four of the ATP Finals with a match to spare.
The 21-year-old, the first Greek player to qualify for the elite eight-man season-ender, backed up his opening round-robin win over Daniil Medvedev with another virtuoso performance at the O2 Arena and is yet to lose a set.
His emphatic victory secured a top-two place in the group and left Zverev, world number one Rafael Nadal and Medvedev scrapping to join him in Saturday's semi-finals.
Earlier, Rafael Nadal produced an incredible fightback to beat Russian Daniil Medvedev and keep his ATP Finals hopes alive on Wednesday, saving a match point at 1-5 down in the decider before winning 6-7(3), 6-3, 7-6(4).
Djokovic faces Roger Federer on Thursday in a must-win match for both players.
A year ago Tsitsipas was winning the NextGen title in Milan -- the ATP's showcase event for up-and-coming players -- and in his two matches so far in London he has bolstered the impression that he has the game to start collecting Grand Slam titles.
His swashbuckling all-court game, with a booming serve, aggressive groundstrokes, deft volleys and silky movement have made him an instant crowd favourite at the O2 Arena.
All those weapons were on display against Zverev in what was the youngest, in terms of combined age, match at the ATP Finals since Andy Murray played Juan Martin del Potro in 2009.
Zverev's last three matches at the O2 were straight sets victories over Roger Federer, Novak Djokovic and Nadal -- a trio with 55 Grand Slam titles between them.
But the form of Monday's group-stage win over Nadal deserted him as Tsitsipas, with whom he is said to have a frosty relationship, beat him for the fourth time in five matches.
Tsitsipas broke in the eighth game when Zverev opted to follow in a weak second serve and made a mess of a low volley.
From that point on it was one-way traffic.
Zverev's level tailed off alarmingly in the second set and Tsitsipas wrapped things up with an ace.
Tsitsipas later revealed he had needed a painkilling injection in a foot injury.
Yet despite having already reached the semis, he will not take his foot off the gas against Nadal on Friday when he could knock the Spaniard out.
"I'm going to give it my all. I'm going to try to give my soul, and I think it's also a very good challenge for me in order to prepare for more difficult matches in the semi-finals," he said. "It's going to educate me, and I'm going to try to get and absorb as much as I can from that."
